1 Three-dimensional characterisation of sedimentary heterogeneity and its impact on subsurface flow behaviour through the braided-to-meandering fluvial deposits of the Castissent Formation (late Ypresian, Tremp-Graus Basin, Spain) Josep M. Puig a, *, Patricia Cabello b , John Howell c , Pau Arbués b a Independent Researcher, Barcelona, Spain b Departament de Dinàmica de la Terra i de l’Oceà, Geomodels Research Institute, Facultat de Ciències de la Terra, Universitat de Barcelona, c/Martí i Franquès s/n, 08028, Barcelona, Spain c Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Old Aberdeen, AB24 3UE UK * Corresponding author. E-mail addresses: josepuig20@gmail.com (J.M. Puig), pcabello@ub.edu (P. Cabello), john.howell@abdn.ac.uk (J. Howell), pau.arbues@ub.edu (P. Arbués). ABSTRACT Fluvial deposits of the Castissent Formation (late Ypresian) form part of the Eocene infill of the Tremp-Graus piggyback basin. The Castissent Formation has been subdivided into three complexes (A, B, and C). This project focuses on the proximal fluvial-sandstones of Complex A in the Mas de Faro outcrop (NE Iberian Peninsula). There, three amalgamated channel belts are exposed in a 15 to 24 m thick succession that passes upwards from sandy braided-river deposits (A1-A2) to coarse-grained meandering-river deposits (A3). Sedimentary heterogeneities within these deposits are characterised and their impact on oil recovery in a reservoir analogue are estimated using field descriptions, virtual outcrop interpretation, a 3D geocellular facies model and fluid-flow simulations. Petrophysical values of different facies types were derived from well data through analogous fluvial deposits of the Eiriksson Formation, in the Norwegian North Sea. Facies analysis show a waning succession of gravel channel lags, unit-bar and dune deposits in A1; a basal mud-clast channel lag, unit-bars and dune deposits in A2, and a coarse-grained point-bar succession in A3. Flow simulation shows: a) fingering of the waterfront related to the gravel channel lag and unit-bars of A1 and A2, and the gravel bars, scour-and-fill deposits and dunes of A3, which act as thief zones hindering efficient sweeping of the intervals immediately above; b) segregation of the injected fluids towards the base of the channel belts due to the fining-upwards successions; and c) vertical